Optical fiber amplifiers have been employed in telecommunication systems to amplify light transmission signals. Optical amplifiers, and more particularly erbium doped fiber amplifiers, are attractive for telecommunications systems for a number of reasons including wavelength independence and bit rate transparency.
Optical amplifiers are disposed along the transmission path of an optical communication system. These amplifiers include at least an amplifying fiber, a coupler, and a pump source used to excite the amplifying fiber. The amplifying fiber is doped with a rare earth element (e.g. erbium) that is excited by light emitted from the pump source to amplify the signals propagating through the amplifying fiber. The pump light usually has a wavelength of 980 or 1480 nm. When a transmission signal, using having a wavelength in the 1550 nm range, propagates through the amplifying fiber, this light stimulates the erbium atoms to release their stored energy as additional 1550 nm light waves which continues as the transmission signals propagates through the amplifying fiber.
Optical amplifiers are attractive because they exhibit low noise, provide a relatively large bandwidth, which is not polarization dependent, and provide low insertion loses at the transmission signal operating wavelengths in the 1550 nm range. The pump light used to excite the amplifying fiber can be configured to co-propagate or counter-propagate with respect to the direction of propagation of the transmission signal. The couplers used to provide the pump light to the amplifying fiber have a high coupling ratio at the pump wavelength and a low coupling ratio at the signal wavelength.
Within an optical communication system, amplifiers are normally configured in pairs, since the optical transmission signals are bi-directional. Because of this, the pump sources are shared among the amplifiers in both directions. These pump sources are the only active component in the amplifier and are usually the most expensive. In telecommunication systems where high reliability is a requirement, for example in undersea systems, redundant pump configurations are necessary to compensate for pump failures. However, some redundant configurations employ additional pump sources that increase costs and power requirements.
Accordingly, there is a need for a fault tolerant amplifier configuration that employs optical pump sources capable of compensating for failed pumps in either signal transmission direction.